1. Field of the Invention
This invention relates to thermoplastic compositions or thermoplastic aggregates having a very pulverized mineral material content.
2. Discussion of the Background:
Generally, and in keeping with the former practice, the preparation of thermoplastic granules or "master blends" has long been known. These are formed of a mineral material and/or pulverized organic substance, such as, for example, very finely ground calcium carbonate, kaolin, talc, aluminum hydrate, titanium dioxide, barium sulfate, zinc oxide . . . , possibly with various agents such as stabilizers, lubricants, plasticizers, cross linking agents, biocide agents, flame retardants, and of a thermoplastic polymer, which is generally a polyolefin with index of fluidity lower than 50, the mixture of these various constituents being subjected to thermal and mechanical actions of malaxation and extrusion.
To the present day, the professional has sought to alter the composition of the granules, increasing as much as possible and for economic reasons the proportion of mineral and/or organic material by using the most powerful malaxators. Today, despite numerous studies in this field, it is recognized that the portion of mineral matter introduced may not exceed 75% by weight of the granules, representing 100 parts by weight of polymer, without producing major inconveniences for the preparation and/or use of the resulting granules ("master blends"), of which the consequence in a nonhomogeneous redispersion of the mineral matter in the redispersion polymers.
One of the major inconveniences which appears during use of mineral matter in elevated amount, near the aforesaid limits, is manifested by heterogeneity (regardless of the importance of malaxation) of the mixture formed from the mineral fill and the polymer, such as a polyolefin, which may result in production of granules with very heterogeneous composition, detrimental to their subsequent utilization. Often, the heterogeneity causes a decline in the mechanical characteristics of the plastics filled with such granules.
Another inconvenience, equally serious, occurs when the professional attempts to increase beyond measure the quantity of mineral substance in relation to the quantity of thermoplastic polymer used in the mixture, i.e., when he attempts to introduce more than 300 parts by weight of mineral fill per 100 parts by weight of polymer, for in this case the result upon malaxation of the constituents, depending on the type of malaxator used, is either an increase in mass of the mixture, producing a blockage of the malaxator with this is, e.g., a screw type, or a segregation of the constituents, manifested by inability to bind them, when the malaxator is the helical or rotor type.
Thus, it becomes inconceivable to use, i.e., to redisperse such heterogeneous compositions in redispersion thermoplastic polymers, without running the risk of producing plastics filled in very irregular manner, whose surface appearance and certain mechanical characteristics would be very markedly impaired, if not compromised.
Hence, it is confirmed that the composition of granulates in mineral matter, relative to the quantity of thermoplastic polymers, is naturally limited by virtue of the aforesaid serious disadvantages.
Accordingly, such difficulties have heretofore led the professional to search for any improvement that could be applied to the various constituents of these plastic compositions so that they can hold increasingly more mineral substances without producing the said disadvantages.
That is why many solutions have been published in the specialized literature to alleviate these serious disadvantages.
First, because mineral materials often have a hydrophilic nature, they have little compatability with any polymer, especially the readily hydrophobic ones. In the case of the readily hydrophobic polymers, it is known that the presence of hydrophilic mineral substances, in the quantities mentioned in the former practice, may result in deterioration of the mechanical characteristics of the filled polymers. To counteract this, the specialized literature has recommended suppressing the hydrophilic nature of mineral substances such as finely ground natural calcium carbonates by treating them with organic substances, forming an envelope compatible with the polymers.
In this way, natural calcium carbonates have been treated with agents chosen among the saturated or unsaturated carboxylic acids of medium or high molecular weight, such as butyric, lauric, oleic, and stearic acid, as well as agents among the alcohols of high molecular weight, transformed by combination into sulfonates, sulfates, or other compounds (FR 1 047 087). Yet is has been ascertained that use of calcium carbonate modified by such treatment does not allow increasing the quantity of mineral substance in the polymers to an appreciable extent.
Subsequently, it was proposed to create good compatability between the hydrophilic mineral substance and the hydrophobic polymer by introducing, during the preparation of the composition, a cross-linking agent to create a bridge between the mineral filler and the polymer, such agent being an organophosphorus compound of phosphonate or phosphonic acid type, such that the thermoplastic composition intended for grinding contains 90 to 20 parts by weight of a polyolefin with a low index of fluidity, 10 to 80 parts by weight of an alkaline earth carbonate, and 0.1 to 10 parts by weight of the cross-linking agent, relative to the mineral filler (DE 2735160 (=US 4,174,340)). However, it was found that use of such agent does not produce the anticipated benefits. In other words, the compatability between the hydrophilic mineral filler and the hydrophobic polyolefins is not sensibly improved, so that certain mechanical properties of the filled polymers still remain unsatisfactory.
It has also been proposed, in order to improve the compatability between the hydrophilic mineral substance and the hydrophobic polymer, to substitute for the aforesaid cross-linking agent an organosulfur compound, of the sulfuric ester or sulfonic acid type, or derivates thereof (EP 0 017 038 (=US 4,307,009)). But, while it has been found that use of this latter agent improves certain mechanical properties of the filled polymers, by the cross-linkage effect, for the recommended quantities of mineral substance, on the other hand the quantity of mineral substance remains limited to 80% by weight of the total mass at most, or to 400 parts by weight of mineral substance per 100 parts of the polymer and the cross-linking agent taken together.
In the same spirit, and to improve the operations of injection or extrusion molding of polymer compositions filled with more than 30% by weight (relative to the total mass) of mineral material, filled thermoplastic compositions have also been proposed (EP -A- 0 114 724), comprising an olefin polymer with index of fluidity (MFI) between 0.1 and 40, a mineral filler (30 to 90% by weight, relative to the total filling), and a so-called "processing aid", formed from a mixture of: (a) a metal salt of a carboxylic acid containing at least 6 carbon atoms and (b) esters of the said acid (a) or an organic phosphate or a lubricator.
But it has been found experimentally that the aforesaid polymer compositions (where the polymer has an index of fluidity between 0.1 and 40) cannot contain more than 80% by weight of mineral fill, without displaying a very disagreeable redispersion in the thermoplastic polymers and, accordingly, a decline in the mechanical properties, despite the presence of the so-called "processing aid."
It has also been proposed (US 4,455,344) to prepare granulates comprising:
(a) 60 to 90 parts by weight of a mineral fill with mean dimension comprised between 0.005 and 100 .mu.m,
(b) 5 to 35 parts by weight of a crystalline polyolefin of mean dimension comprised between 150 and 1000 .mu.m,
(c) 5 to 35 parts by weight of a linking agent, having a melting point at least 10.degree. C. lower than that of the crystalline polyolefin.
To achieve such granulates, the proposed process consists in covering the crystalline polyolefin and/or the particles of the mineral fill with the linking agent, which forms an envelope assuring cohesion of the particles.
But it has been experimentally verified that the granulates prepared by the said process cannot sustain a concentration of mineral substance greater than 80% by weight, without producing a medicore redispersion in the polymers, even when using a mineral substance of medium high granulometry, such as 50 .mu.m. This limit of 80% by weight becomes absolutely inaccessible and unrealistic as a result of a phase segregation which appears during the malaxation when the granulometry of the mineral filler is chosen within mean values below 50 .mu.m, i.e., when the user employs a mean granulometry on the order of, e.g., 3 .mu.m.
As the applicant has ascertained, such process does not permit a coherent paste-like mixture, that is to say, a mixture of identical composition throughout at the production temperature and with the means employed, but instead produces incoherent agglomerates, i.e., such with a composition generally different from one to another and with irregular dimensions, ultimately resulting in poor redispersion in the appropriate polymers.
Finally, with the purpose of improving specifically the mechanical characteristics of compositions made from thermoplastic polymers, compositions have been proposed (EP -A- 0 100 066) based on polar copolymers of the ethylene/vinylacetate type, with index fluidity between 0.1 and 400, preferably 0.1 to 50 (Standard ASTM D 1238, 190.degree. C. -2.16 kg, diestock 2.09 mm), and/or polyolefin polymers.
These compositions also contain 40 to 90% by weight of mineral fillers (the density of which must be taken into account), relative to the total mass, and 0.05 to 5% by weight of surfactants, familiar to the professional, and may also contain agents such as plasticizers, thermoplastic natural rubbers, and size.
But while these polymer comonomer based filled compositions are very suitable for creation, by direct transformation, of soundproofing covers for automobiles, having specific mechanical properties, such is no longer the case when the desired end is not improvement of the mechanical properties, but the production of very heavily filled granulates ("master blends"), designed to be redispersed in a homogeneous monomer in all thermoplastic polymers.
For despite the presence of the surfactant and the use of a considerable percentage of polar polymer with high index of fluidity (MFI 400, e.g., for the aforesaid polymer), the presence of the polar comonomer does not enable production of homogeneous granulates having more than 80% by weight of mineral filler, such as can later be redispersed uniformly in redispersion polymers.
The same disadvantages are found when the polymer compositions contain only olefin polymers whose indexes of fluidity are below 10, as is noticeable in the application examples.